As one means for linking two a.c. systems, a d.c. transmission system is used because of the merits of cost and other characteristics. Such a d.c. transmission system, a bi-polar transmission system with a neutral line (conductor), as shown in FIG. 1, is known. Roughly speaking, the system of FIG. 1 is such that two a.c. systems 20 and 25 are linked through d.c. transmission lines 5 and 7 and a neutral line 6. Two sets of power converters 1 and 3 are provided on the a.c. system 20 side, and two sets of power converters 2 and 4 are provided on the a.c. system 25 side. The a.c. terminals of the power converters 1 and 3 are connected to the first a.c. system 20 respectively through transformers 21 and 23, and the a.c. terminals of the power converters 2 and 4 are connected to the second a.c. system 25 respectively through transformers 22 and 24. The d.c. transmission line 5 constitutes a transmission line of a first pole, the d.c. transmission line 7 constitutes a transmission line of a second pole, and the neutral line 6 constitutes a line common to the both poles. Between the power converters 1 and 2, the first pole the power converter 1, a d.c. reactor 8, the d.c. transmission line 5, a d.c. reactor 9, the power converter 2, and the neutral line 6, is formed. Further, between the power converters 3 and 4, the second pole comprised of including the (converter 3, the neutral line 6, the power converter 4, a d.c. reactor 11, the d.c. transmission line 7, and a d.c. reactor 10 is formed. Respective power converters 1 to 4 can be operated both as a rectifier and as an inverter. Ordinarily, one end B of the neutral line 6 is grounded on the a.c. system 20 side serving as a sending end. The other end A of the neutral line 6 is not grounded. Power converters 1, 2, 3 and 4 are operated and controlled as a rectifier or an inverter by converter control units 12, 13, 14 and 15, respectively. A current command value I.sub.dp1 is delivered from a current command output circuit 16 to the converter control units 12 and 13, and a current command value I.sub.dp2 is delivered from the current command output circuit 16 to the converter control units 14 and 15. Such a control circuit is described in U.S. Pat. No. 4,578,743 as background of the invention.
Explanation will be given in connection with a typical operating mode in the d.c. transmission system of the bi-polar configuration shown in FIG. 1, such that the power converters 1 and 3 are caused to be operated as a rectifier and the power converters 2 and 4 are caused to be operated as an inverter in order to send power from the first a.c. system 20 to the second a.c. system 25. In this case, since a voltage on the d.c. transmission line 5 of the first pole is positive, the first pole is called a positive pole, depending upon circumstances. Further, since a voltage on the d.c. transmission line 7 of the second pole is negative, the second pole is called a negative pole, also depending upon circumstances. In Japan, in order to avoid the problem of an electrical contact, etc. of the underground installed equipment (service water pipe, etc.) by a ground current in the case of communication trouble or an earth return path system, the system of grounding one end of the neutral line 6 as described above is the main current. In other countries, however, there are many instances where the receiving end A is also grounded to constitute an earth return path to thereby omit the neutral line.
Assuming now that the power converters 1 and 3 carry out a rectifier operation, and the power converters 2 and 4 carry out an inverter operation, voltage/current characteristics of respective poles are set as shown in FIGS. 2A and 2B. In FIGS. 2A and 2B, the X-axis and the Y-axis represent a d.c. current I.sub.d and a d.c. voltage V.sub.d, respectively. When it is assumed that the power converters 1 and 3 are respectively operated with constant current characteristics C.sub.1 and C.sub.3 and the power converters 2 and 4 are respectively operated with constant voltage characteristics C.sub.2 and C.sub.4, the operating points of the both poles are intersecting points X.sub.1 and X.sub.2 of the both characteristic lines, respectively. As a result, a d.c. current I.sub.d1 flows in the first pole, and a d.c. current I.sub.d2 flows in the second pole. At this time, a current .DELTA.I.sub.d =Id.sub.1 -I.sub.d2 indicative of a difference therebetween flows in the neutral line 6. It is to be noted that, while constant current control values smaller by a current margin .DELTA.I.sub.dp than constant current control values of the power converters 1 and 3 in the rectifier operation mode are set at the respective power converters 2 and 4 in the inverter operation mode, the constant current control of the power converters 1 and 3 effectively functions in an ordinary state, but the constant current control function of the power converters 2 and 4 is ineffective. In this instance, consideration is taken such that it is not until the d.c. current value lowers by the current margin .DELTA.I.sub.dp that the constant current control of the power converters 2 and 4 becomes effective.
Here, at the first pole, for example, in the case where a current varies in a direction of increasing a current command value, the voltage/current characteristic of the power converter 1 of the first pole changes or shifts to the characteristic indicated by broken lines C.sub.11 of FIG. 2A. Followed by this, the constant current characteristic of the power converter 2 shifts in a direction of increasing the current command value by the same value as above. A new operating point of the first pole is the intersecting point X.sub.11 of the constant current characteristic line C.sub.11 and the characteristic line C.sub.2.
Let us now suppose the case where, at the first pole, the power converter 1 carries out a rectifier operation and the power converter 2 carries out an inverter operation in the same manner as stated above, whereas, at the second pole, the power converter 4 on the a.c. system 25 side carries out a rectifier operation and the power converter 3 on the a.c. system 20 side carries out an inverter operation unlike the above. In this case, at the first pole, a tidal current from the a.c. system 20 side toward the second a.c. system 25 side takes places, and at the second pole, a tidal current from the second a.c. system 25 side toward the first a.c. system 20 side takes place. Namely, the system will be operated by tidal currents in directions different from each other with respect to two poles. Such an operating state is generally called a power sending back operation. In the power sending back operation, tidal current in different directions take place with respect to the both poles. A difference between transmission powers of the both poles serves as an actual transmission power between a.c. systems 20 and 25. This power sending back operation is used for carrying out, between both a.c. systems, power of a value smaller than minimum operating powers of individual power converters for a frequency correction of the a.c. system, or for other reasons.
The voltage/current characteristics of respective poles at this time are as shown in FIGS. 3A and 3B, respectively. In FIGS. 3A and 3B, power converters 1 and 4 are operated in conformity with the constant current characteristics C.sub.1 and C.sub.5, and the power converters 2 and 3 are operated in conformity with the constant voltage characteristics C.sub.2 and C.sub.6. Accordingly, the operating points of the both poles are intersecting points X.sub.1 and X.sub.3 of the both characteristic curves, respectively.
Let now consider the behavior in the case where a short-circuit trouble takes place between d.c. transmission lines 5 and 7 under the state where a power sending back operation is carried out in the circuit of FIG. 1.
In FIG. 4, a flow of currents in the case where a short-circuit takes place between the d.c. transmission lines 5 and 7 under the state where the power converters 1 and 4 carry out a rectifier operation and the power converters 2 and 3 carry out an inverter operation is indicated by broken lines. By occurrence of this short-circuit, between the power converters 1 and 3, there is formed a first current loop 18 in which a d.c. current I.sub.s1 flows through the power converter 1 (in the rectifier operation mode), the d.c. reactor 8, (a portion of) the d.c. transmission line 5, a short-circuited portion 17, (a portion of) the d.c. transmission line 7, the d.c. reactor 10, and the power converter 3 (in the inverter operation mode); and, between the power converters 4 and 2, there is formed a second current loop 19 in which a d.c. current I.sub.s2 flows through the power converter 4 (in the rectifier operation mode), the d.c. reactor 11, (a portion of) the d.c. transmission line 7, the short-circuited portion 17, (a portion of) the d.c. transmission line 5, the d.c. reactor 9, and the power converter 2 (in the inverter operation mode).
By formation of these current loops, the operating points of the power converters 1 to 4 are caused to be newly in conformity with the voltage/current characteristics shown in FIGS. 5A and 5B unlike the case of FIGS. 3A and 3B. Namely, in the first current loop 18, since the power converter 1 is operated by the constant current characteristic C.sub.1, and the power converter 3 is operated by the constant voltage characteristic C.sub.6, the operating point X.sub.4 is determined by the both characteristics C.sub.1 and C.sub.6. On the other hand, in the second current loop 19, since the power converter 4 is operated in accordance with the constant current characteristic C.sub.5, and the power converter 2 is operated in accordance with the constant voltage characteristic C.sub.2, the operating point X.sub.5 is determined by the both characteristics C.sub.2 and C.sub.5.
In the case where transmission powers are equal to each other, i.e., d.c. currents are equal to each other with respect to the both poles before such short-circuit failure takes place, even if a short-circuit as described above takes place on the d.c. transmission lines, there is no change in voltage/current of respective operating points of FIGS. 3 and 5, so any change does not substantially take place in currents flowing in respective d.c. transmission lines. For this reason, a current differential relay and/or a current directional relay for detecting changes in a current flowing direction, etc., generally provided at d.c. transmission lines cannot detect a short-circuit failure as described above.
Here, in the case where only a current command value of the first pole is increased, constant current control values of the voltage/current characteristics of the power converters 1 and 2 shift in directions indicated by broken lines C.sub.12 and C.sub.21 of FIGS. 5A and 5B, respectively. In this case, a new operating point of the first current loop 18 in FIG. 4 changes to X.sub.6 (FIG. 5A). On the contrary, in the second current loop 19, the current command value shifts in an increasing direction as indicated by broken lines C.sub.21 in FIG. 5B by the power converter 2. However, the operating point X.sub.5 determined by the both characteristics C.sub.2 and C.sub.5 is not changed, so the operating point still remains at the point X.sub.5. In the state of the voltage/current characteristic of FIG. 5B, since it becomes impossible to ensure a current margin .DELTA.I.sub.dp of a predetermined value between power converters 4 and 2, which is determined by the characteristic curves C.sub.5 and C.sub.21, it is easy for constant current control by the power converter 2 in the inverter operation mode to be carried out. As a result, any interference in control may take place between the both power converters 4 and 2. Here, when it is assumed that the current command value of the first pole is increased, the constant current control value indicated by broken lines in FIG. 5B further shifts in a direction indicated by an arrow in FIG. 5B, so the operating point cannot be ensured. Eventually, there is the possibility that the system may be down with respect to the both poles, and/or any power converter may be damaged, so its bad influence will be exerted on the a.c. system.
Accordingly, it is required to take any measure to promptly and securely detect a short-circuit failure between d.c. transmission lines in the case where a power sending back operation is carried out in a bi polar d.c. transmission system thus to prevent in advance inconveniences such as system down, etc.